In meteor scatter communications systems, it is advantageous to employ two-way communications, so as to acknowledge when a meteor scatter message was received at the base from a mobile terminal and so as to control communications within a network. As before stated, the current invention involves a two-way mobile communication system based on a synergistic use of the Loran broadcasts and of such meteor scatter communications, wherein the Loran navigation signal transmissions are supplemented with a communication capability for transmitting a small amount of information at a time; and wherein such communication capability can be used to broadcast to many users, and be generally available without waiting for specific propagation conditions unlike in prior meteor scatter systems. The availability of a return signal via meteor scatter communication from remote sites also permits a central site ("the base" or "base station") to receive position data from the remotes and determine what position corrections are necessary and to monitor and display the location of the mobiles.
It is well known that the outbound Loran broadcast from a group of synchronized Loran base stations can be used to allow mobile stations, or more generally remote stations distributed over an area, receiving the Loran signal, to fix their position from the knowledge of the positions of the fixed base Loran sites and from the differential times of Loran signal arrival. Suitable transmitters for Loran-C broadcasting are described, for example, in U.S. Pat. Nos. 3,711,725; 3,889,263; 4,001,598; 4,151,528; and 4,423,419 of one of the common assignees of the present invention; and typical suitable receivers, as for mobile users (boats, aircraft, ground vehicles), in U.S. Pat. Nos. 3,736,590; 3,774,210; 3,921,076; 4,392,138; and 4,482,896.
Not so well known, but now proven, is the fact that the Loran navigation signal broadcasts can also be modified simultaneously to communicate data outbound to the mobile users by suitable supplemental modulating of the Loran navigation signal to send up to between, for example, 20 and 100 bits per second of communications data to ranges of up to 1000 kilometers without affecting the basic navigational function of the Loran signal. Such techniques are described in U.S. Pat. Nos. 4,800,391 and 4,821,038 of said common assignee.
While such communications use of the Loran broadcast is limited, it is, for example, entirely adequate to advise a particular mobile terminal, in a specified network of terminals, that a meteor scatter link is to be established, or to acknowledge successful receipt of its message to the base via meteor scatter. This new capability permits the base station to address mobile terminals individually and in groups and obviates previous problems associated with the control of meteor scatter communications between a mobile terminal in the net and the base. Without such outbound communication, mobile terminals have to follow a very strict protocol of who reports when, in order to avoid having several mobile terminals reporting at the same time, thus interfering with one another.
In turn, meteor scatter communications from mobile terminals can be used to provide an inbound link to the fixed Loran sites, which Loran communications alone, being one-way (outbound), cannot provide. Thus, in accordance with the present invention, a mobile terminal can readily report its location and status to the meteor scatter base station, which can thus track the reporting terminals. With the present invention, the inbound link becomes more efficient, because of the availability of acknowledgment by Loran, particularly since the base may not be able to use the same meteor trail outbound to acknowledge the receipt of the inbound message. In addition, meteor scatter communications can now also be used for a particularly efficient transfer of short messages from one user outbound to other mobile users in the same net via the base.